1. MQTT State transitions
Andrew Banks

2. MQTT V3.1.1 State transitions
Void
Connect
Connack returns SessionPresent=false
Session Exists
(connected)
Connect
cleanSession = false
Connack returns SessionPresent=true
Disconnect
from cleanSession=false
Network break
from cleanSession=false
Disconnect
from cleanSession=true,
or Session State is empty.
Network break
from cleanSession=true
or Session State is empty
Send Will
Session Exists
(disconnected)
Send Will
Connect
cleanSession=true
(and creates a NEW session)
Void

3. MQTT V5 State transitions
Void
Connect
Connack returns SessionPresent=false
Session Exists
(connected)
Connect
cleanStart = false
Connack returns SessionPresent=true
Disconnect
Network break
Send Will, if Will on Session expiry NOT set
Session Exists
(disconnected)
Session Timer, from Connect or Disconnect
Session State exists, even if empty.
Connect
cleanStart=true
(and creates a NEW session)
Void
Send Will, if Will on Session expiry IS set

4. Proposal
The DISCONNECT packet has an optional Session State Expiry Interval, if present this overrides the value set on CONNECT.
Clean Session is renamed to Clean Start, this causes any existing Session State to be deleted on CONNECT but not on DISCONNECT.
A new flag SendWillOnSessionStateExpiry, is sent as a CONNECT optional value. If absent or false the will message is sent on network disconnection where no DISCONNECT packet is processed as today. If true the Will message is sent on Session Expiry, regardless of how this occurs. Or
A new flag WillDelay, is sent as a CONNECT optional value. If absent the will message is sent on network disconnection where no DISCONNECT packet is processed as today. If present the Will message is sent after the specified interval so long as no new CONNECT is processed first.
The Server retains knowledge of the Session for the duration of Session Expiry, even if the Session Contains no Messages or Subscriptions.

5. SendWillOnSessionStateExpiry vs SendWillDelay
WillDelay must be less than SessionStateExpiryInterval, otherwise you get a zombie Will message with no associated Session State.
This rule is hard to enforce if the SessionStateExpiryInterval is reset on DISCONNECT.